The present invention relates to a toner for the development of an electrostatic image for use in the development of an electrostatic latent image formed by electrophotographic process or electrostatic recording process with a developer, a process for the preparation thereof, an electrostatic image developer, and a process for the formation of an image.
A method for rendering an image data visible via electrostatic image such as electrophotography has been used in various fields. In electrophotography, an electrostatic latent image is formed on a photoreceptor at a charging/exposure step. The electrostatic latent image is developed with a developer containing a toner, and then processed at a transferring step and a fixing step to become visible.
Known examples of the developer to be used in electrophotography include two-component developer comprising a toner and a carrier and one-component developer comprising a magnetic or non-magnetic toner alone. Such a toner is normally prepared by a knead-grinding method which comprises melt-kneading a thermoplastic resin with a pigment, a static controller and a release agent such as wax, cooling the mixture, finely grinding the mixture, and then classifying the particles. In this method, inorganic or organic particles may be attached to the surface of the toner particles to improve the fluidity or cleaning properties of the toner. These methods can produce an extremely excellent toner but has the following some disadvantages.
In the ordinary knead-grinding method, the resulting toner has an indefinite shape and surface structure that closely change with the grindability of the material used or the grinding conditions. Thus, it is difficult to intentionally control the shape and surface structure of the toner. In the ordinary knead-grinding method, the range of selection of material is restricted. In other words, the resin dispersion of colorant must be brittle enough to be finely ground by an economically possible grinding machine. However, when the resin dispersion of colorant is rendered brittle to meet these requirements, the resulting toner can further produce a fine powder or have a change in shape due to mechanical shearing force applied to the toner in the developing machine. For the two-component developer, fine powder is fixed to the surface of carrier to accelerate the deterioration of chargeability of the developer. For the one-component developer, the expansion of particle size distribution causes the toner particles to be scattered. Further, the change in the shape of the toner particles causes a deterioration of developability that can lead to deterioration of image quality.
In the case where a large amount of a release agent is added to prepare a toner, the kind of the thermoplastic resin to be used in combination with the release agent can often affect the exposure of the release agent to the exterior of the toner. In particular, when a slightly less grindable resin having an enhanced elasticity given by the incorporation of a high molecular component and a brittle wax such as polyethylene are combined, the polyethylene is drastically exposed to the exterior of the toner. This is favorable for releasability of fixed image during fixing or removability of untransferred toner from the photoreceptor. However, since the polyethylene in the surface layer can easily migrate when acted upon by mechanical force, the developing roll or photoreceptor can be easily stained with the carrier, resulting in the drop of reliability.
Further, when the shape of the toner particle is indefinite, sufficient fluidity cannot be secured even if provided with a fluidizing aid. Thus, when acted upon by mechanical shearing force during operation, the particles on the surface of the toner migrate into the indentation on the toner, deteriorating the fluidity thereof with time, or become embedded in the interior of the toner, deteriorating the developability, transferability and cleaning properties thereof. Further, when the toner recovered at the cleaning step is returned to the developing machine for reuse, the resulting image quality can be deteriorated even more easily. When a fluidizing aid is added more to prevent these defects, the generation of black peppers on the photoreceptor or scattering of particles of fluidizing aid can easily occur.
In recent years, as a method for controlling the shape or surface structure of the toner particle there has been proposed a method for preparing a toner using emulsion polymerization aggregation as disclosed in the Unexamined Japanese Patent Application Publication No. Sho 63-282752 and the Unexamined Japanese Patent Application Publication No. Hei 6-250439. In accordance with this method, a particulate resin dispersion is usually prepared by emulsion polymerization method or the like. Separately, a colorant dispersion having a colorant dispersed in a solvent is prepared. The two dispersions thus prepared are then mixed to form aggregated particles having a diameter corresponding to that of the desired particulate toner. The dispersion is then heated so that the aggregated particles undergo coalescence to prepare a toner. In this manner, the shape of the toner particles can be controlled to some extent, making it possible to improve the chargeability and durability of the toner. However, since the particulate toner thus prepared has a substantially uniform internal structure, it remains disadvantageous in that the fixed sheet cannot be fairly peeled off at the fixing step and the image outputted on OHP sheet can not be provided with a stable transparency.
Thus, in order that the toner can invariably maintain its properties in electrophotographic process even under various mechanical stresses, it is important to prevent the release agent from being exposed to the exterior of the toner or enhance the surface hardness of the toner with out impairing the fixability thereof as well as enhance the mechanical strength of the toner itself and secure sufficient chargeability and fixability.
Further, there has been a growing demand for the enhancement of image quality in recent years. For the formation of color image in particular, there has been a remarkable tendency for the reduction of diameter of toner particles for the purpose of attaining highly fine image. However, when the diameter of toner particles is reduced leaving the conventional particle size distribution as it is, the presence of fine powder adds to the problem of staining on the carrier or photoreceptor or scattering of toner, making it difficult to attain high image quality and high reliability at the same time. In order to prevent this defect, it is also important to provide a sharp particle size distribution and reduce the particle diameter of toner.
In a digital full-color copying machine or printer, a color image original is subjected to color separation through various filters, i.e., B (blue) filter, R (red) filter and G (green) filter. The resulting latent images made of dots having a diameter of from 20 .mu.m to 70 .mu.m corresponding to the original image are then developed utilizing subtractive mixture process with various developers, i.e., Y (yellow), M (magenta), C (cyan) and Bk (black) developers. However, since this process requires that a large amount of developers be transferred and dots having a reduced diameter be coped with as compared with the conventional black-and-white copying machine or printer, it is even more important to secure uniform chargeability, high durability, sufficient toner strength and sharp particle size distribution. Further, taking into account the tendency for the enhancement of operation speed of these machines and the reduction of energy consumption, an even higher low temperature fixability is required. In this respect, too, coalescence process, which is suitable for the production of particulate material having a sharp particle size distribution and a small particle diameter, has been paid attention. For full-color machines, it is important to securely mix a large amount of toners. It is also essential to improve the color reliability of the resulting image and secure high OHP transparency.
On the other hand, as a release agent component for preventing low temperature offset during fixing there is usually incorporated a polyolefin-based wax. Combined with this measure, a slight amount of a silicone oil is uniformly applied to the fixing roller to improve high temperature offset resistance. As a result, the transfer material thus outputted has some of the silicone oil attached thereto, giving uncomfortable sticking during handling.
The Unexamined Japanese Patent Application Publication No. Hei 5-61239 proposes a toner adapted for oilless fixing comprising a large amount of a release agent component incorporated therein. Although the incorporation of a large amount of a release agent gives some improvement of releasability, the binder resin component in the toner and the release agent become compatibilized with each other, making it difficult to secure stable or uniform running of release agent and hence obtain stable peelability. Further, since the cohesive force of the binder resin in the toner depends on the weight-average molecular weight or glass transition temperature of the binder resin, it is difficult to directly control the stringiness and cohesiveness of toner during fixing. Moreover, the free release agent component can cause malcharging.
As approaches for solving these problems there are proposed a method which comprises adding a high molecular component to provide the binder resin with rigidity in the Unexamined Japanese Patent Application Publication No. Hei 4-69666 and the Unexamined Japanese Patent Application Publication No. Hei 9-258481 and a method which comprises introducing chemically crosslinked structure to compensate for the lack of rigidity, thereby reducing the stringiness of toner at the fixing temperature and hence improving peelability during oilless fixing, in the Unexamined Japanese Patent Application Publication No. Sho 59-218460 and the Unexamined Japanese Patent Application Publication No. Sho 59-218459.
As disclosed in the Unexamined Japanese Patent Application Publication No. Sho 59-218460 and the Unexamined Japanese Patent Application Publication No. Sho 59-218459, mere addition of a crosslinking agent to a binder resin causes a rise in the viscosity, i.e., cohesive force of the toner during melting, increasing the rigidity of the binder resin itself and hence making it possible to lessen the temperature dependence of peelability during oilless fixing or carried amount of toner to some extent. However, it is difficult to provide the fixed image with a sufficient surface gloss at the same time. Further, the bending resistance of the fixed image is insufficient. As disclosed in the Unexamined Japanese Patent Application Publication No. Sho 59-218460, when the molecular weight of the crosslinking agent is increased, the molecular weight of the entanglement region increases, enhancing the flexibility of the fixed image itself somewhat. However, it is difficult to balance between elasticity and viscosity properly. As a result, the desired temperature dependence of peelability during oilless fixing and temperature dependence of carried amount of toner and the desired surface gloss of fixed image and OHP transparency can hardly be attained at the same time. In particular, satisfactory fixed image cannot be obtained with energy-saving type fixing device and high printing speed type copying machine or printer.